1. Field of the Invention
The present invention relates to a laser scanning optical apparatus, and more particularly to a laser scanning optical apparatus to be employed for a color image forming apparatus such as an electrophotographic copying machine, a printer or the like.
2. Description of Related Art
In recent years, full-color electrophotographic image forming apparatuses, such as copying machines and printers, are generally of a tandem type, wherein four photosensitive drums for Y (yellow), M (magenta), C (cyan) and K (black) respectively are arranged in parallel, and four color images formed on the respective photosensitive drums are transferred onto an intermediate transfer member to be combined with each other.
Japanese Patent Laid-Open Publications No. 2003-5113 (reference 1) and No. 2004-226497 (reference 2) suggest a structure for a laser scanning optical apparatus to be employed for an image forming apparatus of this tandem type. In the structure suggested by these publications, four beams are converged on a single polygon mirror and concurrently deflected (so-called one-side deflection method). Accordingly, in the scanning optical system, four optical paths are formed, and the scanning optical system has a common scanning lens which is commonly used for the four optical paths and four exclusive scanning lenses which are used exclusively for the respective optical paths. The distances between the respective exclusive scanning lenses and the respective corresponding photosensitive drums are equal to one another.
Japanese Patent Laid-Open Publication No. 2000-180749 (reference 3) discloses that optical paths have mutually different magnifications in a sub-scanning direction and that the last optical elements (optical elements closest to the respective photosensitive drums) in the respective optical paths are plane mirrors.
FIG. 6 generally shows the structure of a laser scanning optical apparatus suggested by the references 1 and 2. In this laser scanning optical apparatus 100, laser beams deflected by a polygon mirror 101 pass through a common scanning lens 102 and are reflected by plane mirrors 104Y, 104M, 105M, 104C, 105C, 104K and 105K, respectively. Then, the laser beams pass through exclusive scanning lenses 103Y, 103M, 103C and 103K and irradiate photosensitive drums 110Y, 110M, 110C and 110K, respectively.
Such an image forming apparatus of the tandem type is often used in a monochromatic print mode, and therefore, a toner hopper 111K in a black image forming unit is larger than toner hoppers 111Y, 111M and 111C in the other image forming units. Additionally, a refuse toner bottle (not shown) in the black image forming unit is larger than those in the other image forming units.
In the laser scanning optical apparatus 100 shown by FIG. 6, it is a premise that the distances between the last optical elements in the respective optical paths and the corresponding photosensitive drums, namely, the distance between the exclusive scanning lens 103Y and the photosensitive drum 110Y, the distance between the exclusive scanning lens 103M and the photosensitive drum 110M, the distance between the exclusive scanning lens 103C and the photosensitive drum 110C and the distance between the exclusive scanning lens 103K and the photosensitive drum 110K are equal to one another. Therefore, when the black image forming unit is designed to be larger than the other image forming units, a dead space D is made between the laser scanning optical apparatus 100 and the image forming units, which results in an increase in the size of the image forming apparatus.
When plane mirrors with no optical powers are arranged as the last elements in the respective optical paths, the magnifications in the sub-scanning direction of the respective optical paths are naturally large, and the common scanning lens must be wide. This results in a rise in cost and becomes a bar to division into the optical paths. This problem will be described later with reference to FIG. 5.